1. Field of the Invention
This invention relates to an illuminating optical system for transferring a predetermined pattern, and in particular to a mask illuminating optical system for transferring an integrated circuit pattern from a mask to a wafer.
2. Description of the Prior Art
In recent years, in accordance with the requirement for higher density and higher speed, the dimensions of elements included in semiconductor integrated circuits have tended to become minute and the line width of elements transferred onto wafers has decreased from several microns to 1-2 microns and further to the order of sub-microns.
As mask pattern printing methods, there are the contact method in which superposed printing is effected with a mask and a wafer being brought into intimate contact, the proximity method in which a mask is illuminated with the mask and a wafer being spaced apart from each other by several microns to several tens of microns, the projection method in which the pattern on a mask is transferred to the surface of a wafer by the use of a projection optical system, and the step-and-repeat method. Of these methods, the contact method and the profimity method still are those mainly used as the methods of printing semiconductor elements for mass production because of their high throughput or number of elements produced per unit time and the completeness of the printing apparatus.
The resolving power of the contact method is most excellent among the above-mentioned printing methods. However, this method suffers from a problem in achieving intimacy of contact which results from warping of the wafer and mask and non-uniformity of application of the resist layer, and also is liable to cause the film surface to be injured due to the direct contact between the mask and the wafer, thus reducing the yield of production. Particularly, as the pattern becomes more minute, even a slight injury of the film surface may become a fatal defect and therefore, the use of the contact method for the transfer of a minute pattern would suffer from numerous problems. On the other hand, the proximity method in which a mask and a wafer are kept out of contact causes no injury of the film surface of the mask, whereas in this method, the diffraction created between the mask and the wafer adversely affects the image printed and the resolving power is deficient.
The minimum line width W of the pattern image by the proximity method is given by ##EQU1## (iEEE, ED-28, No. 11, 1268-1278), where S is the distance between the mask and the resist surface and .lambda. is the printing wavelength. It is seen from equation (1) that to increase the resolving power achieved with the proximity method, the wavelength may be shortened or the gap S may be made smaller. However, due to the warping of the wafer and mask, the gap cannot be made smaller than a minimum amount and it is desirable to shorten the printing wavelength. In fact, the wavelength used in the proximity method has become shorter, i.e., from the so-called UV light of 365 nm-436 nm to the Deep UV light of 250 nm-290 nm. However, with a super high pressure mercury lamp or a xenon mercury lamp used as a light source, the energy of the Deep UV area is small as compared with the energy of the UV area, and the sensitivity of photoresist is low in the Deep UV area. Therefore, in printing using the Deep UV light, much exposure time is required and the throughput or number of elements produced per unit time is low.
To efficiently effect pattern transfer in the proximity method, the UV light may be used as the printing wavelength for a rough pattern which does not require a high resolving power and the Deep UV light may be used as the printing wavelength for a pattern which requires a high resolving power. This has heretofore been effected by the use of two types of illuminating optical systems having different printing wavelengths, i.e., a UV illuminating optical system and a Deep UV illuminating optical system.